This invention relates generally to a heat and pressure fuser for an electrophotographic printing machine, and more particularly the invention is directed to release agent application methods and apparatus therefor.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent image on the photoconductive member. The toner attracted to a donor roll is then deposited on a latent electrostatic images on a charge retentive surface which is usually a photoreceptor. The toner powder image is then transferred from the photoconductive member to a copy substrate. The toner particles are heated to permanently affix the powder image to the copy substrate.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser Roll (NFFR) fuser, the heated fuser roll is provided with a layer or layers that are deformable by a harder pressure roll when the two rolls are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll. In a Nip Forming Pressure Roll (NFPR) fuser the pressure roll is provided with a deformable outer layer which is deformable by the harder fuser roll.
The heated fuser roll is usually the roll that contacts the toner images on a substrate such as plain paper. In any event, the roll contacting the toner images is usually provided with an abhesive (low surface energy) material for preventing toner offset to the fuser member. Three low surface energy materials which are commonly used for such purposes are PFA (PerFluoroAlkoxy resin), VITON.TM., a fluoroelastomer based on the copolymer of vinylidene fluoride and hexafluoropropylene, and silicone rubber. All of these materials, in order to maintain their abhesive qualities, require release agents specific to the material.
Following is a discussion of prior art, incorporated herein by reference, which may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description to follow, may provide a better understanding and appreciation of the present invention.
Heretofore, the methods of using release agent materials for preventing offset of toner material to contact fuser members has been to apply a coating of release agent material to the heated fuser member which contacts the toner images. This has been accomplished as described in the prior art in various ways.
RAM systems of the prior art are, in one way or another, subjected to excessive amounts of heat. This is primarily because they have to operate in close proximity to a large source of heat such as the heated fuser roll. Such RAM systems result in various shortcomings such as: the RAM acting as a thermal drain on the fuser roll thereby causing the phenomena known as droop at cold and hot startup, release agent degradation due to exposure to high temperatures, nonuniformity of oil application between copy 1 and copy N, necessity of using high operating temperature and highly abrasive materials for the metering blade, unnecessary heat dissipation into the machine and undesirable amounts of release oil on substrates such as plain paper etc.
Donor/metering roll Release Agent Management (RAM) systems have been used as part of a roll fuser apparatus for some time. Such a RAM system is disclosed in U.S. Pat. No. 4,214,549 issued on Jul. 29, 1980 to Moser. This patent illustrates a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising a thermoplastic resin. The apparatus includes an internally heated, fuser roll cooperating with a backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The pressure roll is the softer of the two rolls, therefore, the nip is formed by the harder fuser roll indenting the softer pressure roll. The heated fuser roll is characterized by an outer layer or surface which by way of example is fabricated from a silicon rubber or a fluoroelastomer such as VITON.TM. to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump from which it is dispensed by means of a metering roll and a donor roll, the former of which contacts the release fluid in the sump and the latter of which contacts the surface of the heated fuser roll.
U.S. Pat. No. 4,770,116 granted to Rabin Moser on Sep. 13, 1988 relates to a heat and pressure roll fusing apparatus for fixing toner images to copy substrates, the toner comprising a thermoplastic resin. The apparatus includes an internally heated fuser roll cooperating with an unheated bare metal backup or pressure roll to form a nip through which the copy substrates pass with the images contacting the heated roll. The heated fuser roll is characterized by a conformable outer layer or surface which by way of example is fabricated from a silicone rubber or a fluoroelastomer such as VITON.TM. to which a low viscosity polymeric release fluid is applied. Release fluid is contained in a sump and the pressure roll is partially immersed in the fluid. Thus, the release fluid is applied to the surface of the internally heated fuser roll via the bare metal pressure roll. The roll structures are such as to provide maximum area of contact in the nip, while minimizing the area of contact between the pressure roll and the copy substrates.
U.S. Pat. No. 5,392,105 granted to Rabin Moser on Feb. 21, 1995 relates to a applying a release agent material which is solid at room temperature. The release agent material is contained in a sump for application indirectly to a heated fuser roll structure. A pressure roll structure contacts the solid release agent material and a source of heat energy serves to elevate the solid release agent material to its operating temperature which is well below the operating temperature of the heated fuser roll. A metering blade contacting the pressure roll structure causes the liquefied release agent material to be metered to the desired thickness.
U.S. Pat. No. 3,716,221 granted to Gorka et al on Feb. 13, 1973 relates to a fusing device for fusing thermoplastic resinous particulate material to a receptor sheet. The fusing device includes a fusing roller having a resilient fusing blanket supported on the periphery thereof and heating means to heat the fusing blanket to a temperature sufficient to fuse the particulate material. A backup roller is urged toward engagement with the deformable fusing blanket to press the receptor sheet carrying the particulate material into contact with the fusing roller. The fusing roller is coated with an off-set preventing liquid which is applied thereto from the backup roller at predetermined intervals during operation of the fusing device.
U.S. Pat. No. 4,309,957 granted to Joseph A. Swift on Jan. 12, 1982 relates to a improved fluid applicator wick for use in applying release fluids to a fuser member surface of a fusing system for fusing toner images is described. The wick comprises a working surface material which contacts the fuser member surface, and a backing material to which the working surface material is needled. A preferred fluid applicator wick comprises a layer of felt or fiber made from a tetrafluoroethylene fluorocarbon polymer, sold under the trademark TEFLON.TM., as a working surface material which is needled to a fibrous or felted aramid material, such as that sold under the trademark NOMEX.TM.. One side of the wick contacts a release agent or fluid supply means such as an applicator roll, a spray of release fluid, a reservoir of release fluid.